In the art of water cooled internal combustion engines wherein an axial flow fan is supported for rotation adjacent a water cooling radiator and is driven by the engine, it is common to use a clutch between the fan and the engine drive shaft. The actuation of the clutch is controlled in response to the temperature of either the air flowing through the radiator or the cooling liquid being circulated through the engine and radiator so that the engine is operated within a predetermined temperature range. For example, U.S. Pat. No. 3,584,716 discloses a viscous fluid clutch which mounts on a drive shaft and incorporates a rotary valve member actuated by a bimetallic temperature sensing element for controlling the circulation of the fluid within the clutch.
Another form of clutch which has been used to drive an engine cooling fan, is disclosed in a 1974 publication No. 74-DE-12 of the American Society of Mechanical Engineers, entitled "A New Wet Clutch Fan Drive System". This drive system incorporates a stationary shaft which is supported by a bracket and on which is mounted both a rotary clutch housing driven by V belts and a rotary fan support hub. The clutch housing is connected or coupled to the fan support hub by a series of interfitting clutch discs which are operated in cooling oil, and the oil is circulated through an external heat exchanger. The series of clutch discs are actuated between clamped and released positions by axial movement of a non-rotating piston supported by the stationary shaft. Fluid to actuate the piston is controlled by a valve which operates in response to movement of a temperature sensing element such as a wax type heat motor.
In the operation of a "wet" clutch such as disclosed in the above mentioned publication, it is desirable to have a forced or positive flow of oil outwardly between the relatively rotating clutch plates or discs to create an "oil shear" action, especially during the time the clutch discs are being compressed together or engaged. Thus wearing of the discs is substantially eliminated, and the clutch is provided with dependable operation over a substantially long period of service time. It is also desirable for such a clutch to be of construction and operation so that both the construction cost and the maintenance cost are minimized.
Furthermore, with respect to cooling fans used in conjunction with large horsepower engines, for example, such as installed on buses and trucks, it is frequently desirable to provide for driving the fan either at the full speed of the engine with no slip between the fan and the engine shaft or to stop the fan so that it has no rotation. For example, when the engine is being operated on a cold day, and the cold air is sufficient to cool the engine, it is desirable to prevent even "windmilling" of the fan at a few hundred rpm.